Developmental changes produced in the retinofugal pathways of rats and ferrets by early monocular enucleations: the effects of age and the differences between normal and albino animals

Abstract

Early monocular enucleations were done in rats, either at embryonic day 16 (E16) or on the day of birth, and the surviving uncrossed pathway was studied either at birth for some of the animals enucleated prenatally, or in the adult for all of the other animals. The uncrossed pathways were studied by using HRP as a retrograde tracer. The neonatal enucleations showed the increase of the surviving uncrossed component previously documented by others. In contrast to this, a prenatal enucleation produced a significant reduction in the surviving uncrossed pathway at birth. If these animals survived to be adults, then the surviving uncrossed pathway was slightly increased relative to normal. We conclude that two quite distinct mechanisms have been exposed by these experiments, one acting prenatally and producing a reduction in the uncrossed pathway, and the other acting postnatally and producing an increase. The postnatal effect, which is due to a decrease of the normally occurring ganglion cell death, thus neutralizes the prenatal effect, so that the most effective demonstration of the prenatal effect is to be seen before the period of cell death (early postnatal in rats and ferrets). The same methods were applied to prenatal ferrets at E26-E28 and, in order to see the maximum prenatal effects, the uncrossed pathways were studied at birth in all of these animals. There was a severe reduction of the uncrossed pathway throughout, and this was greatest in the animals with the earliest enucleations. Since the uncrossed pathway in normally reared albino animals is abnormally small, the effects of an early prenatal enucleation in albino rats and ferrets were compared with the effects in normally pigmented animals in order to determine whether the early enucleation was producing an abnormality comparable to the albino abnormality. Prenatal enucleations reduced the uncrossed pathway not only in normally pigmented but also in albino neonatal rats and ferrets. Further, the characteristic position of the nasal border of the temporal retina, which is abnormal in albino animals, was unaffected by the enucleation in either the albino or the pigmented animals, except where, in ferrets, enucleations produced a complete loss of the temporal concentration of ipsilaterally projecting ganglion cells (the temporal crescent). The earlier enucleations showed a greater tendency to produce such a complete loss of the temporal crescent. We conclude that the developmental mechanisms affected by the early enucleations are distinct from those that act to produce the albino abnormality even though both produce an abnormally small uncrossed pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

Visual abnormalities in albino wallabies: A brief note

“…That is, the relative area of the albino uncrossed bundle would have been less than the 2.9% (about 2.4%) of the total cross-sectional area of the chiasm. The ratio of uncrossed bundle size in the albino animal to uncrossed bundle size in normally pigmented animals (about 27%) is comparable to differences reported for eutherian mammals in terms of numbers of ipsilaterally projecting ganglion cells ( 20-30%; see Thompson and Morgan, 1993;Chan and Guillery, 1993). In eutherians, the uncrossed component does not form a discrete bundle anywhere; its fibres are mingled throughout their course with one or the other of the crossed components.…”

“…In the retina the gene action appears to affect the ganglion cells, but when the axons of these cells reach the chiasm, the axons must take their appropriate pathway on the basis of local interactions. From the evidence of the normal chiasmatic structure, and from the evidence of the effects of very early enucleations (Godement et al, 1987;Guillery, 1989;Taylor and Guillery, 1995a;Chan and Guillery, 1993), it would appear that in carnivores and rodents, axons do not form a permanent part of the uncrossed pathway unless they first encounter fibres from the other eye. In contrast to this, in marsupials, evidence about the chiasmatic pathway structure and about the effects of very early enucleations indicates that the determination of the uncrossed fibre path is independent of the crossed fibres from the other eye (Mendez-Otero et al, 1986;Taylor and Guillery, 1995b).…”

Developmental modifications of olivocerebellar topography: The granuloprival cerebellum reveals multiple routes from the inferior olive

“…It is also consistent with early olivocerebellar targeting by cell-surface molecules (Chédotal et al, 1997;Plagge et al, 2001). Second, the regression of an ipsilateral path is widely observed during maturation of other projection pathways (O'Leary and Stanfield, 1989;Thompson et al, 1993Thompson et al, , 1995Chan and Guillery, 1993). Although the function of transient ipsilat-erally projecting axons remains unclear, it has been proposed that, because of their early outgrowth, they act as pioneers, which direct later-arriving axons (Marcus and Mason, 1995).…”

Expression of chondroitin sulfate proteoglycans in the chiasm of mouse embryos

“…Retinal axons have an overall increase in pause frequency and a reduction in growth rate as well as an extensive remodeling of growth cone morphology at the chiasm. These characteristic growth behaviors of retinal axons may be caused by a contact-mediated interaction with cellular elements in the chiasm, such as the radial glial cells and chiasmatic neurons (Sretavan et al, 1994;Mason and Sretavan, 1997), and with axons from the other eye (Godement et al, 1990;Chan and Guillery, 1993;. Moreover, the cells of the chiasm may produce soluble factors that act to slow down the growth of retinal axons and prompt the axons for other guidance signals within the chiasm (Wang et al, 1996).…”